Like many people I wasn't up on what normal levels of radiation exposure are, but some quick digging gave some very illuminating answers. So, here's what you're exposed to on an annual basis:You Receive ~620 Millirem Per YearAccording to stats from the US Nuclear Regulatory Commission average yearly exposure is roughly 620 millirem--half of which comes from natural sources (cosmic radiation, from the soil, radon, etc) and half comes from manmade sources. Note that geography can play a big part in that. In Colorado, for example, natural radiation exposure can be 1000 mrem per year due to higher altitude.

Some perspective: NRC limits "occupational radiation exposure to adults working with radioactive material to 5,000 mrem per year" and anything below 10,000 mrem is considered a low dose of radiation. In the Chernobyl disaster the 134 people working in the plant to put out the fire received 80,000-1,600,000 mrem--28 of them died within three months of exposure.

Back to what's going on in Japan: LA Times reports that the aircraft carrier that moved in response to detecting elevated radiation levels experienced exposure equal to what a person might receive in one month from natural sources, so perhaps 26 mrem, and that the contamination was "easily removed with soap and water."

Yesterday, TreeHugger reported that exposure in Japan was equal to about one year of natural radiation (Christine explained that this was about 35-40 chest x-rays)--so perhaps 310 mrem.

UPDATE: Newer reports of radiation levels at the front gate of the Fukushima power plant place them significantly higher than earlier information--at that location radiation equivalent to three years of normal exposure in one hour have been detected.

UPDATE 2:BBC News reports that "radiation dosages of up to 400 millisieverts per hour were recorded at the site" and notes "A single dose of 1,000 millisieverts causes temporary radiation sickness such as nausea and vomiting."

On differing units used in measuring radiationThough the USNRC uses millirems and variants in discussing radiation exposure, the current internationally preferred unit is the sievert and its subdivisions of millisievert and microsievert. For conversion: 1 millisievert = 100 mrem.